This invention relates to wafers of single crystal compound semiconductors belonging to groups III-V.
Compound semiconductors belonging to groups III-V comprise, for example, GaAs, GaP, InP, InAs and GaSb. Compound semiconductors of this type are used in various devices, such as for example, light emitting devices, light sensitive devices and high-speed operational devices.
It is very important in a manufacturing process to be able to distinguish the front and rear surfaces of a compound semiconductor wafer. Therefore an arrangement which enables one to distinguish these surfaces from one another is highly desirable.
An ingot of a single crystal of compound semiconductor of groups III-V is grown by a pulling method or a boat method. The single-crystal ingot is then sliced into many thin wafers. The sliced wafers are polished on one or both surfaces. On a wafer many active electronic devices may be formed by a sequence of steps. Then, the processed wafer is scribed and cut many times vertically and horizontally into many small chips each individual chip being an electronic device unit.
These compound semiconductors have a crystalline structure of zinc blende type (ZnS), which belongs to cubic system on crystallography. The crystalline point group is denoted by 4 3 m. The crystalline point group does not have four-fold symmetry, i.e. the front surface of the wafer sliced from the crystalline ingot is not identical with the rear surface. When electronic devices are formed on a wafer of the compound semiconductor, it is important to be able to distinguish which surface is the true front surface and which is the mesa-direction on the front surface.
Cleavage planes are generally denoted by (110) planes. The numerals 0 or 1 in the round parentheses are known as the plane index. The general representation includes all planes which can be obtained by the symmetry operations of the point group starting from a (110) plane.
For example, on the wafer whose front surface is denoted by the (100) plane, the cleavage planes are four planes (011), (011), (011) and (011) which meet at right angles with the front surface of the wafer. The upper line drawn upon a numeral signifies a minus sign in crystallography.
In case of the wafer in which the front surface is the (100) plane, a cleavage plane is parallel with a perpendicular defined on the front surface. A small bow-shaped part can be cut off from a circular wafer along a cleavage plane. The cord generated by cutting off the bow-shaped part, which is parallel with one of the four cleavage planes, denotes a crystallographic direction. Thus the cord is called as an "orientation flat".
The orientation flat designates one of four cleavage planes. However, in the case of a compound semiconductor, a single orientation flat is insufficient to identify the crystallographic direction, because the crystal of the compound semiconductors lacks four-fold symmetry and the line &lt;100&gt; is a four-fold-width-inversion symmetry axis. The crooked parentheses denotes a crystallographic line, which is perpendicular to a plane signified by the same numerals bracketed by a set of round parentheses.
Among the four cleavage planes which are perpendicular to the surface of a (100) wafer, two cleavage planes which are parallel to one another are equivalent. But, two cleavage planes which are perpendicular to each other are unequivalent.
Two different kinds of cleavage planes must be clearly distinguished. It is inappropriate to make any electronic devices on a wafer without knowing which direction is the mesa-direction. The term "mesa-direction" will now be clarified.
When the surface of a wafer is etched by a proper etchant along the cleavage planes, the sectional shape etched along a cleavage plane becomes a normal trapezoid whose bottom is wider than the top. But the sectional shape etched along another cleavage plane becomes a reversed trapezoid whose bottom is narrower than top. The former section etched is called "mesa", and the latter section is called "reverse-mesa".
The etching process to check the mesa direction will now be briefly explained. Photoresist solution is painted on a wafer to be examined. A mask plate having a rectangular transparent part (or opaque part) is put on the resist-painted wafer, the sides of the rectangle of the mask coinciding with the cleavage planes of the crystalline wafer. The masked wafer is then exposed by a light and the wafer is developed.
A small rectangular resist layer stays on the wafer surface. Then the wafer is etched in an appropriate etchant. The wafer surface is gradually etched except for the rectangular part covered by the photoresist layer. The sectional shapes of the periphery of the rectangular part are significant.
Two parallel sides of the four sides of the rectangular part have a sectional shape like a normal trapezoid with a wider bottom. Another two parallel sides which are perpendicular to the first mentioned sides have a sectional shape like a reverse trapezoid with narrower bottom. The former sectional shape is called "mesa". The latter sectional shape is called "reverse-mesa".
In the etching process, the photoresist layer has four sides parallel with the cleavage planes (0 .+-.1 .+-.1). In the vicinity of the four sides of the photoresist layer etching makes progress along intermediate angles held between the (100) and (0.+-.1 .+-.1) planes. Therefore equivalent-progress-etching-planes are four planes denoted by the plane symbol (1 .+-.1 .+-.1).
If the same etching process is executed also upon the rear surface of the wafer, the equivalent-progress-etching-planes are eight planes denoted by (.+-.1 .+-.1 .+-.1).
"l", "m" and "n" signify either of the two integers +1 and -1. We consider how the (l m n) plane is converted by the symmetry operations of 4 3 m. An assembly of planes which are converted by the operations into another planes belonging to the same assembly forms an independent group of the planes.
The crystalline point group notation 4 3 m is shortly explained. "4" signifies four-fold-width-inversion symmetry. The rotation axes are &lt;.+-.100&gt;, &lt;00 .+-.1&gt; and &lt;0 .+-.10&gt;. The n-fold symmetry means the rotation of 360/n degrees around some pertinent axis does not change the crystal structure. "3" signifies three-fold symmetry around &lt;111&gt;, &lt;111&gt;, . . . . axes. "m" signifies the mirror symmetry with regard to (110), (011), . . . planes.
The eight planes denoted by (lmn) can be classified into two groups, in which the plane indices satisfy the equations EQU (i) l m n=1 EQU (ii) l m n=-1
We designate the front surface of wafer as (100) and the rear surface as (100). However the designation to which surface the minus sign is attributed is an alternative. By determining the notation of another two axes beside the designation of front or rear surface, the distinction between the two groups (i) and (ii) of planes are done with a definite meaning.
In the etching process, the etching planes are represented by (1 .+-.1 .+-.1), because the etching was done along the periphery of a small rectangular part having four sides parallel with the cleavage planes.
According to the mentioned classification of (i) and (ii), (111) plane is equivalent with (111) plane, and (111) plane is equivalent with (111) plane. But (111) plane is unequivalent of (11) plane.
Because two groups of planes are unequivalent, the etching speeds on the two group of planes are reasonably different, where the etching speed is faster, the etched part is likely to be a reverse-mesa shape. Where the etching speed is slower, the etched part becomes a mesa shape. Therefore distinction between mesa and reverse-mesa directions becomes clear. The mesa direction is a well defined direction on a surface. The reverse-mesa direction is also a well-defined direction on it.
If (111) plane and (111) plane form a mesa shape, (111) plane and (111) plane form a reverse-mesa shape.
If we project the crystallographical directions on the surface of the wafer, the projected mesa direction is denoted by a line. The projected reverse-mesa direction is also denoted by a line which is perpendicular to the projected mesa direction.
FIG. 6 is a schematic perspective view of a wafer more clearly showing the mesa directions and reverse-mesa directions on both surfaces. The wafer, as shown, is in a state after etching.
The rectangular portion covered by the photoresist is not etched away but remains on the surfaces. The sides of the rectangular portion are parallel with the cleavage planes.
Around the periphery of the photoresist layer, the edge parts on the mesa direction M are etched in a shape with a wide bottom and a narrow top. On the contrary the edge parts on the reverse-mesa directions R are etched in a shape with a narrow bottom and a wide top. The projected reverse-mesa direction on the surface is perpendicular to the mesa direction.
To facilitate the understanding of the etching shape slant broken lines are drawn in the bottom portions of the photoresist-covered layer at the same level of other surface part.
If the photoresist layer F is a square, the bottom portion of the photoresist-covered layer is a rectangle which is longer in the mesa direction and shorter in the reverse-mesa direction.
As FIG. 6 clearly shows, in the rear surface of the wafer the mesa direction and the reverse-mesa direction are inverted. The mesa direction projected on the rear surface which a dotted R denotes is parallel with the reverse-mesa direction M projected on the front surface. The wafer is sliced in (100) plane. If we assume l=1 on the front surface, l=-1 on the rear surface.
On the front surface, &lt;111&gt; and &lt;111&gt; are mesa directions. Therefore on the rear surface, &lt;111&gt; and &lt;111&gt; are mesa directions.
The projected mesa directions M on the front surface are &lt;011&gt; and &lt;011&gt;. The projected reverse-mesa direction R thereon are &lt;011&gt; and &lt;011&gt;.
The projected mesa directions M on the rear surfaces are &lt;011&gt; and &lt;011&gt;. The projected reverse-mesa direction R on the rear surface are &lt;011&gt; and &lt;011&gt;.
It is an important feature of group III-V semiconductors that the projected mesa direction and reverse-mesa direction are inverted on both surfaces. This is because the compound semiconductors have the crystalline structure of the zinc blende (ZnS) type. In a unit cell of lattice structure group III atoms form a first face-centered cubic sub-lattice. Thus a denotes a lattice constant. Three dimensional coordinates x,y,z are defined on a lattice. The positions of the group III atoms are (.+-.a/2,0,0), (0, .+-.a/2,0), (0,0,.+-.a/2) and (.+-.a/2, .+-.a/2, .+-.a/2). In the same coordinates group V atoms form a second face-centered cubic sub-lattice which deviate from the group III atoms' sub-lattice by (a/4,a/4,a/4). Thus the positions of group V atoms are (la/4, ma/4, na/4), where the product l m n is either 1 or -1. As group V atoms are not identical to group III atoms, two face-centered cubic sub-lattices are distinguishable.
The crystalline structure of silicon (Si) is a diamond type, which is akin to the zinc blend type except the clear distinction of group III and group V atoms.
If we replace both group III atoms and group V atoms of Si atoms, we obtain the crystalline structure of silicon. In the case of Si the first sub-lattice and the second sub-lattice which deviates (a/4,a/4,a/4) from the first sub-lattice are totally equivalent, because both sub-lattices are formed only with silicon atoms.
In a group III and V semiconductor a group III atom is connected with the nearest four group V atoms by covalent bonds. The vector representations of the four covalent bonds are denoted by (la/4,ma/4,na/4), where l, m and n are +1 or -1, and the product l m n is either 1 or -1.
If the product l m n is +1 for four vectors projected from a group III atom, the another product l m n must be -1 for four vectors projected from a group V atom. Thus mesa direction differs from the reverse-mesa direction in group III-V semiconductor.
However in crystalline silicon, two sub-lattices are equivalent, because all the lattice sites are occupied by Si atoms. The vector represntation (la/4,ma/4,na/4) are applicable to both equivalent sub-lattices in which the product l m n is either 1 or -1. Thus the mesa direction coincides to the reverse-mesa direction. And cleavage plane is denoted by (111). Thus in the case of silicon semiconductor even concepts of mesa direction don't exist. All cleavage planes in crystalline silicon are totally equivalent.
The reason why we must distinguish the mesa direction and the reverse-mesa direction on a surface on compound semiconductor will now be explained.
Epitaxy, impurity doping, etching, evaporation coating and other wafer processes are carried out on a wafer for producing FETs or other electronic devices. When some parts of a front surface of a wafer are etched to make electrodes by evaporation coating process, the electrodes must be formed to cover the step parts generated by the former etching process.
If the step part is a slant edge on the mesa direction, the electrode is well formed on the step part, because thin electrode layer coats on a gently sloping area of the surface.
However, if an electrode might be formed on a reverse-mesa edge which has a bottom part narrower than a top part, the electrode layer does not contact on the edge tightly.
Hence metallic electrodes must be formed on the slant edge etched in the mesa shape. This is one reason why the mesa direction on a wafer should be clarified. There are additional reasons why we must distinguish mesa direction or reverse-mesa direction on a front surface of a wafer. But further explanation of the reasons is omitted, because it is not the purpose of the invention.
In order to designate the crystallographical directions, not only the cleavage planes but also the mesa direction should be denoted. Conventional designation of the directions on a compound semiconductor wafer uses two distinguishable orientation flats with different lengths which are perpendicular with each other.
Two orientation flats are shaped by cutting the (100) wafer periphery along the cleavage planes, that is (011) or (011) plane, and (011) or (011) plane.
Both orientation flats coincide with the cleavage planes, but either main-orientation flat or sub-orientation flat is formed along the mesa direction. Because the lengths of the orientation flats differ from one another, the two orientation flats are distinguishable.
By a glimpse of two orientation flats on a wafer periphery, we can discern which are the cleave directions, which side of the wafer is the front surface, and which is the mesa direction on the front surface.
The conventional designation method of compound semiconductor wafers has some disadvantages. Although the lengths of the orientation flats are different, two orientation flats cannot easily be distinguished.
A practical rule should be described to discern the front surface or determine the mesa direction on the surface. For example, the rule is that if a wafer is positioned in a state wherein the main orientation flat is at the bottom and the sub-orientation flat is on the right side, the surface facing to the operator is the front surface.
Unskillful operators are apt to confuse two orientation flats, because it is difficult to determine the length of cords formed on a periphery of a circle by eye measurement. In addition, the rule to determine the front surface is rather complicated for them.